The invention relates to a pneumatic percussive tool for a paving breaker and/or drill hammer according to the preamble of claim 1.
Amongst currently conventional percussive tool types for drill hammers or paving breakers, the one design which above all has proven to be successful is one in which a drive piston which is designed as a hollow piston is set to perform an oscillating axial movement by way of a crank drive. In the interior of the drive piston which is guided in the housing of the hammer, a solid percussion piston is moved which protrudes at the open end of the hollow drive piston and cyclically influences a chisel tool or an interconnected riveting die. For this purpose, a pneumatic spring is formed in a hollow space between the percussion piston and the drive piston and transmits the forced movement of the drive piston to the percussion piston and drives said percussion piston against the tool.
The percussive tool takes up relatively little installation space and can be produced cost-effectively. Furthermore, the percussion piston achieves a high impact speed. The reliable starting behaviour of the percussive tool from the idling mode is also particularly advantageous.
However, the considerable mass of the drive piston has proven to be disadvantageous, as it is moved in a reciprocating manner by the drive even during the idling mode, i.e. in a state in which the tool does not work any material. The relatively large oscillating masses make it more difficult to handle the hammer during idling.
DE 198 28 426 A1 discloses a pneumatic percussive tool, wherein the drive piston consists substantially of a piston base and a guide sleeve, in which the percussion piston can be moved in a reciprocating manner. The wall thickness of the guide sleeve is very low, whereby the weight of the drive piston is low and the oscillations occurring particularly during idling are small. The guide sleeve is provided with several air compensating slots, through which air is able to penetrate into the pneumatic spring between the percussion piston and the drive piston after each impact, in order to compensate for any loss of air possibly occurring during the impact operation. Furthermore, the guide sleeve is provided with idling orifices which allow a reliable transition from the percussion mode to the idling mode.
Even if, in the case of the pneumatic percussive tool according to DE 198 28 426 A1, the oscillations which occur particularly during idling could be reduced considerably, a further reduction in the mass of the drive piston and thus a corresponding reduction in the idling oscillations would be desired.
Therefore, it is the object of the invention to achieve a further reduction in the vibrations occurring during idling whilst retaining the positive features of the percussive tool.
In accordance with the invention, the object is achieved by means of a pneumatic percussive tool in accordance with claim 1. Advantageous embodiments of the invention are defined in the subordinate claims.
An inventive pneumatic percussive tool in accordance with the preamble of claim 1 is characterised by virtue of the fact that in the idling mode the percussion piston has slid completely out of a front end of the guide sleeve.
As a consequence, during normal percussion mode at least a part of the percussion piston can still be moved axially in the guide sleeve of the drive piston. With respect to the transition to the idling mode, the operator lifts the paving breaker and/or the drill hammer together with the tool from the stones which are to be worked, whereby the tool shaft slides to a certain extent out of the hammer. Accordingly, it is possible for the percussion piston likewise to move further forwards, in the direction of percussion, and slides out of the guide sleeve. Ideally, it moves completely out of the front end of the guide sleeve and is only held by means of the housing of the percussive tool. As a consequence, the hollow space between the percussion piston and the drive piston is opened, so that during further movement of the drive piston it is possible for air to penetrate into the hollow space surrounding the pneumatic spring and it is possible to prevent the percussion piston from being drawn back and to prevent any subsequent percussion operations. This results in a reliable idling behaviour.
If the operator then places the tool on to the stones, the tool shaft is displaced into the interior of the hammer, whereby the percussion piston is then also urged back into the guide sleeve of the drive piston. As a consequence, the hollow space between the drive piston and the percussion piston is closed, so that the effect of the pneumatic spring can be realised and the percussion mode can be recommended.
In the case of a particularly advantageous embodiment of the invention, the axial length of the guide sleeve of the drive piston is smaller than a maximum axial path of the percussion piston between its extreme positions. The axial length of the guide sleeve must be dimensioned in such a manner that although the percussion piston can be moved reliably in a reciprocating manner in the guide sleeve during the percussion mode, the percussion piston must be able to slide completely out of the guide sleeve during the idling mode. This results in a considerable reduction in the axial length of the drive piston and thus in a reduction in its mass and the idling oscillations which are associated therewith.
For the purpose of reliably holding the percussion piston during the idling mode, the percussive tool housing is advantageously provided with a device for receiving the piston head.
The receiving device allows the build-up of negative pressure for the purpose of holding the percussion piston, as will be explained hereinunder.
The movement of the percussion piston can be guided in various ways. It is particularly advantageous if the percussion piston is guided exclusively by the percussive tool housing, e.g. by its piston shaft, both during the idling mode and during the percussion mode. This can be utilised such that in the percussion mode, i.e. when the piston head of the percussion piston is located in the guide sleeve of the drive piston, the guide sleeve is guided by means of the percussion piston but not by the percussive tool housing. In this manner, it is possible to obviate undesired double fits which could occur if both the guide sleeve and also the percussion piston were each guided in the percussive tool housing. It is not necessary to guide the piston head of the percussion piston during the idling mode when the piston shaft is guided to a sufficient extent.
Accordingly, the above described device for receiving the piston head in the percussive tool housing can be dimensioned to be sufficiently large in order to avoid tolerance problems.
In the case of a particularly advantageous embodiment of the invention, the pneumatic spring can be supplied with and relieved of air during the idling mode by way of the front end of the guide sleeve, wherein the guide sleeve does not comprise any further idling orifices for the purpose of supplying the pneumatic spring with air during the idling mode. In contrast, in the case of the prior art, in particular in the case of the aforementioned DE 198 28 426 A1, it is necessary to provide corresponding idling orifices in the guide sleeve, in order to guarantee a reliable idling mode. However, since in accordance with the invention the percussion piston moves completely out of the guide sleeve and thus the end side of the guide sleeve is open, additional idling orifices are not required. The omission of idling orifices means that the costs of producing the drive piston fall and the susceptibility of cracking and fracture is reduced by reason of the omission of the notch effect which is otherwise produced by the idling orifices. Furthermore, it is possible to reduce tolerance problems not least owing to the shorter length of the drive piston.
It is particularly advantageous if the guide sleeve is provided with at least one air compensating slot which comprises an axial length which is longer than the axial length of the piston head of the percussion piston. As a consequence, any air loss in the pneumatic spring which has occurred during the generation of a percussive action can be compensated for whenever the piston head is located at the level of the air compensating slot. Then, for a brief moment the pneumatic spring downstream of the piston head is connected to the surrounding space upstream of the piston head. If the drive piston is already involved in its return movement at this point in time and thus exerts a suction effect upon the percussion piston, the negative pressure in the pneumatic spring will cause additional air to be drawn into the hollow space between the drive piston and the percussion piston. The air compensating slot allows the guide sleeve to be designed with a minimum wall thickness.
A further advantageous embodiment of the invention is that the receiving device which holds the piston head in the percussive tool housing in the idling mode is provided with a one-way valve which connects a front hollow space formed between the receiving device, the piston head and the piston shaft to the area surrounding the percussive tool, e.g. a crank space of the hammer. During the transition from the percussion mode to the idling mode and as the percussion piston slides accordingly out of the guide sleeve into the receiving device, excess air pressure which is provided in an air cushion and which is formed between the percussion piston and the receiving device can consequently be reduced via the one-way valve with respect to the area surrounding the percussive tool. Only if the percussion piston has travelled into the receiving device does the one-way valve close, whereby the attempt by the percussion piston to perform a return movement causes a suction effect to occur which holds the percussion piston in the receiving device. Only in the event of a correspondingly large force, which is produced e.g. when the tool is placed on to the stones which are to be worked, can the percussion piston be urged out of the receiving device and guided back into the guide sleeve.
It is not absolutely necessary for the percussion piston to consist of a piston head and a piston shaft which differs therefrom in geometric dimensions. On the contrary, in the case of a different embodiment of the invention, the piston head and the piston shaft of the percussion piston can also comprise a substantially identical diameter.